The present invention relates in general to taking geometrico-morphological measurements of a subject.
More particularly, the invention relates to a method of determining a geometrical parameter of an eye of a subject.
A particular, but non-exclusive, application of the invention lies in taking geometrico-morphological measurements on a future subject of eyeglasses for the purpose of personalizing the optical design of the corrective ophthalmic lenses that are to be mounted in the frame selected by said future subject.
In order to customize the ophthalmic lens for a specific subject, geometrical, postural and behavioral parameters of the subject and/or of the subject with its eyeglasses need to be determined.
Among these parameters, the position of a center of rotation of at least one of the eye is determined.
Movement of the eye can generally be considered as being a combination of rotations about a particular point that is referred to as the center of rotation of the eye (ERC).
It is desirable to determine the position of this point, for example to perform calculations of a personalized optical design by ray tracing for the corrective lens that is to be fitted to the frame properly.
In present practice, the position of the ERC may be deduced approximately from the position of the cornea by assuming a mean value for the radius of the eye, typically a value of about 15 millimeters (mm). Unfortunately, the radius of the eye varies significantly from one individual to another, such that this approximation leads to significant errors that are highly penalizing for the pertinence of the personalized optical design calculation.
In order to determine the position of this center of rotation, it is also known to capture at least two facial images of the subject equipped with a reference accessory referred to as a position-identification element while the subject looks at the image capture device.
These images are treated in order to determine the center of rotation of the eye. The reference accessory gives information on the relative position of the head of the subject and the image capture device.